The rotational speed of a permanent magnet type motor, for example, a brushless DC motor is dependent on the current supplied to the stator windings of the motor and the magnetic force and impedance of the motor. In a configuration in which the motor is PWM (Pulse Width Modulation) controlled using an inverter circuit, when the voltage supplied to the inverter circuit varies in a state where, with the PWM duty having reached 100%, the rotational speed of the motor is lower than a rotational speed command value, the current supplied to the motor varies and the rotational speed of the motor fluctuates. Namely, the rotational speed of the motor fluctuates due to fluctuations of the supply voltage itself or noise superimposed on the supply voltage.
Generally, when a motor is PWM-controlled, feedback control is performed so as to have a PWM duty determined based on the difference between a given rotational speed command and a detected rotational speed of the motor. In such a control system, however, when the PWM duty reaches 100% whereas the rotational speed of the motor has not reached the rotational speed command value, the rotational speed cannot be further increased and rotational speed control becomes impossible. Technique addressing such an issue is disclosed, for example, in Patent Literature 1.